Geodes
RocksForKids.com
| |
|
|
|
| |
|
|
|
| |
|
|
|
| |
GEODE CHALCEDONY QUARTZ
DESCRIPTION: A geode is a hollow sphere. This hollow geode half is
partially filled in on the inside with creamy-yellow botryoidal
chalcedony.
COLOR(S): light, beige, creamy-yellow
ORIGIN: Halls Gap, Kentucky, USA |
|
|
-
are sphere-shaped stones, usually at least partially hollow, and often lined
inside with sparkling mineral crystals or concentric layers of minerals
-
as river beds erode, geodes tumble into streams
-
 |
|
 |
|
Calcite - Ohio |
|
Dolomite "geode"
Manitoulin Island, Ontario |
 |
 |
 |
Blue Celestite Geode
Madagascar |
|
Dolomite, Strontianite, Marcasite |
 |
 |
 |
| |
|
|
Formation theory #1 - Brazil
- found in ancient volcanic rock filled with cavities produced by steam,
later filled when solutions of seeping water found there way in & formed
quartz crystals or agate
- quartz is the most common material found in geodes
- colour depends on what impurities were picked up by the solutions
- if the solution is deposited slowly & has relatively few impurities,
it results in crystal lined cavities
- if solution is deposited rapidly, cavity is filled with bands of
fine-grained quartz - namely agate
Formation theory #2
- they were originally nodules of limestone or anhydrite
- formed within a soft sediment by concentric outward growth around small
nucleus or core
- the interior of the concretion washed out, causing a hollow
- groundwater solutions then left behind a replacement of the geode walls
& the crystals inside
- takes 240 million years to form a geode
Geode Glossary
source: Dictionary of Geological Terms
| agate |
- a waxy variety of cryptocrystalline quartz (chalcedony) in which the
colours are in bands, clouds or distinct groups
|
| agate nodule |
- a lense-shaped agate nodule
- fill holes can be seen & affect pattern of banding
- rapid deposition of groundwater causes dense agate to form
|
| amygdaloid |
- volcanic rocks that contain numerous gas cavities filled with
secondary minerals such as zeolites, calcite, chalcedony or quartz
- filled cavities are called amygdules
|
| amygdule |
|
| aphanitic |
- pertaining to a texture of rocks in which the crystalline
constituents are too small to be distinguished with unaided eye
- used to describe igneous rock rhyolite
|
| botryoidal |
- chalcedony forms bubbly surface
- found in the geodes of Kentucky & Indiana
|
| chalcedony |
- cryptocrystalline quartz and much chert
- material agate is made of
- has a great array of colours: blue, gray, black, off-white, purple (deccomposition
of iron-bearing minerals)
- in Hall’s Gap geodes
- sulfide mineralization occurs in chalcedony geode
|
| chert |
- a cryptocrystalline variety of quartz
- occurs as nodules, lenses or layers in limestone and shales
|
| concretion |
- a nodular or irregular concentration of certain authigenic
constituents of sedimentary rocks and tuffs
- developed by the localized deposition of material from solution,
generally about a central nucleus
- are solid, grow from the center outward & are generally
noncrstalline though some crystals have been observed
- concretions are formed by the deposition of distinct minerals,
different from the surrounding rock, very firmly cemented around a
nucleus
- most common cementing materials are calcite, siderite & silica
- parts of plants, animals & well-preserved fossils may be found
at the nucleus
- harder than enclosing rock
|
| enhydro |
- an agate with a visible bubble moving around when tilted
|
| floaters |
- fully formed or doubly-terminated crystals that are unattached to
other crystals
- found in geode cavities & also in vugs such as "Herkimer
Diamonds"
|
| geode |
- hollow, globular bodies
- subspherical shape
- clay film between geode wall & the enclosing limestone matrix
which is like a skin or crust
- an outer chalcedony layer
- an interior drusy lining of inward projecting crystals
- evidence of expansion or growth
- slow deposition of groundwater causes crystals to grow
- estimated it takes 240 million years to form
- geodes form either in sedimentary rock or in basalts/andesites
- hollows appeared in sedimentary rocks either because of animal
burrows, tree roots or mud balls
- word "geode" derived from Latin meaning
"earthlike", rounded shape
- found in USA: Montana, Indiana, Iowa, Illinois, Pennsylvania
(Lancaster Co. goethite geodes), Tennessee (Loretto), Oklahoma
(Comanche County etc.), Michigan (Houghton), Dugway geodes, Colorado
(Chaffee County ,Marshall Pass), Hauser Geode Beds, California
- amethyst geodes from Artigas, Uruguay or near Lajeado, Rio Grande Do
Sul, Brazil
-
|
| industrial geodes |
- a.k.a. garden geodes - found in the garden
- solid all the way through, milky quartz
|
| ironstones |
- large concretions of siderite & calcite found in shales
assoicated with coal beds in Kentucky
- name not much in use now
- collected just south of Holland, Michigan
|
| Keokuk Geode |
- a specific type of geode which can be found in a 45 mile radius of
Keokuk, Iowa in the Tri-State area - Iowa, Illinois, Missouri
- geode is Iowa’s official state rock
- most geodes come from strata of the lower Warsaw Formation, a rock
unit of Mississippian age
- formed in sedimentary rock- shale, shaley dolomite & limestone
- original concretions thought to made of limestone or anhydrite which
then dissolved and was relaced by chalcedony for the skin and
interiors filled in over long periods of time
- source of mineralizing water is speculative as quartz is only weakly
soluble
- predominately quartz crystals
- silicates: crystaline quartz, kaolinite
- carbonates: calcite, aragonite, ferroan dolomite, malachite,
smithsonite, stilpnosiderite
- zinc sulfide
- sulphides: iron pyrite, sphalerite, chalcopyrite, millerite
- sulfates: barite, selenite, jarosite
- oxides: goethite, hematite, pyrolusite
|
| lithophysae |
- hollow, bubblelike structures composed of concentric shells of
finely crystalline alkali feldspar, quartz and other minerals
- found in certain silicic volcanic rocks such as rhyolite and
obsidian
- a.k.a. thunder egg in the USA
- in France, come from the Esterel massif in the south-eastern part of
France
- found in Ploand
|
| nodule |
- small more or less rounded body generally somewhat harder than the
enclosing sediment or rock matrix
- is solid
- agate, jasper & chalcedony form in nodules
- other common minerals that occur in nodules are siderite, gypsum,
calcite, quartz and barite/celestite
|
| rhyolite |
- the aphanitic equivalent of a granite
- thunder eggs form in rhyolitic rocks - Oregon & southeastern
California
|
| septarian nodule |
- septarian nodules that have a hollow cavity are sometimes known as
"septarian nodule geodes" or "geodes"
- Utah septarian nodules formed in ancient sea floor during the
Cretaceous period, 50 - 70 million years ago - filled with calcite -
yellow centres (calcite), brown lines (argonite), grey rock
(limestone), white or clear (barite)
|
| septarium |
- a roughly spheroidal concretion, generally of limestone or
clay-ironstone
- cut into polyhedral blocks by radiating and intersecting cracks
which have been filled (and the blocks cemented together) by veins of
some material, generally calcite
- some contain fossils
- a.k.a. Septarian Nodule, Septarian Boulder, Turtle Stone
- presence of an "Avenue of Entry"
- in southern Illinois, they contain fluorite, sphalerite, witherite
& sometimes galena
- in Utah, the type has a different shape & may be filled with
agate
- thunder egg of Oregon is a septaria - cousin
- most geodes coming from France are in fact septaria (Drome, Ardeche,
Hautes-Alpes, Remuzat)
|
| spherulite |
- a small radiating & usually concentrically arranged aggregation
of one or more minerals generally of spherical or spheroidal shape
- formed by the radial growth of acicular crystals in a rigid glass
about a common centre or inclusion
|
| thunder egg |
- geodelike body commonly containing opal, agate or chalcedony
weathered out of welded tuff or lava
- form only in rhyolitic rocks, almost always in association with
perlite
- exterior surface is "warty" & it has a rind
- interior cross-section commonly exhibits a star-shaped outline
- interiors are filled with chalcedony, agate or opal
- when hollow, thunder eggs are sometimes referred to as geodes
- no thunder eggs exist in the USA east of Colorado except for a
single locality near Grand Marais on Minnesota’s North Shore
- is the common name for lithophysae
|
| turtle stone |
- a septarium with distinctive external markings on the outside
resembling the shell of a turtle
|
| vesicle |
- a small circular enclosed space
- a small cavity in an aphanitic or glassy igneous rock, formed by the
expansion of a bubble of gas or stem during the solidification of the
rock
|
| vug |
- a cavity, often with a mineral lining of different composition from
that of the surrounding rock
- can form spherically, in veins, irregular openings
- vugs do not generally weather out of the host rock
|
Geodes: Some Locations & Types
USA
- Indiana, Washington Co. near Salem Indiana
- clear calcite crystals on pink-orange dolomite
- thin walled
- clear calcite crystals on white quartz
- Iowa, Keokuk – Warsaw Strata
- Keokuk geodes are from a 45 mile radius
- borders Iowa, Illinois & Missouri
- botryoidal chalcedony with calcite cubes
- blue & green chalcedony
- pink calcite crystals
- sphalerite
- quartz iron – cheery orange, quartz oxide – cherry red
- dew drop diamond
- pink dog-tooth
- iridescent brown calcite
- industrial (garden bed) geodes – usually solid all the way through
- interior filled with milky coloured quartz
- found in people’s garden’s
- banded agate lining with crystals
- world-famous locality noted for its millerite in small geodes
- light pink amethyst
- Kentucky, South Fork Creek
- bumpy, brown round geodes in river bed
- stepped calcite, spaghetti, smoky quartz, botryoidal chalcedony
- orange-yellow solids, clear, white
- Sheffler Geode Mine – Keokuk Geodes
- gray, smoth skinned round
- clear quartz crystals
Brazil Rio Grande do Sul
- amethyst
- most fantastic geode known – 33 ft. in length, 16 ˝ ft. wide, 10 ft.
high
- estimated weight of seventy thousand pounds
- lined with purple amethyst
- a piece weighing 400 lbs. is in the Smithsonian Institute
Mexico, Chihuahua
- amethyst quartz, smoky, gray, white, lavender and brown quartz
- along with contrasting crystals of calcite, goethite
Uruguay
- adjoining Brazil, Rio Grande do Sol
Canada
- High Falls, Manitoulin Island, Ontario
Geodes
-
are sphere-shaped stones, usually at least partially hollow, and often
lined inside with sparkling mineral crystals or concentric layers of
minerals
-
as river beds erode, geodes tumble into streams
 |
 |
 |
Calcite - Ohio
|
Quartz Geode - Mexico
|
Dolomite "geode"
Manitoulin Island, Ontario
|
 |
 |
 |
Blue Celestite Geode
Madagascar
|
|
Dolomite, Strontianite, Marcasite
|
 |
 |
 |
|
|
|
Formation theory #1 - Brazil
-
found in ancient volcanic rock filled with
cavities produced by steam, later filled when solutions of seeping water found
there way in & formed quartz crystals or agate
-
quartz is the most common material found in geodes
-
colour depends on what impurities were picked up by the solutions
-
if the solution is deposited slowly & has relatively few impurities,
it results in crystal lined cavities
-
if solution is deposited rapidly, cavity is filled with bands of
fine-grained quartz - namely agate
Formation theory #2
-
they were originally nodules of limestone or anhydrite
-
formed within a soft sediment by concentric outward growth around small
nucleus or core
-
the interior of the concretion washed out, causing a hollow
-
groundwater solutions then left behind a replacement of the geode walls
& the crystals inside
-
takes 240 million years to form a geode
Geode Glossary
source: Dictionary of Geological Terms
| agate |
- a waxy variety of cryptocrystalline quartz (chalcedony) in which the
colours are in bands, clouds or distinct groups
|
| agate nodule |
- a lense-shaped agate nodule
- fill holes can be seen & affect pattern of banding
- rapid deposition of groundwater causes dense agate to form
|
| amygdaloid |
- volcanic rocks that contain numerous gas cavities filled with
secondary minerals such as zeolites, calcite, chalcedony or quartz
- filled cavities are called amygdules
|
| amygdule |
|
| aphanitic |
- pertaining to a texture of rocks in which the crystalline
constituents are too small to be distinguished with unaided eye
- used to describe igneous rock rhyolite
|
| botryoidal |
- chalcedony forms bubbly surface
- found in the geodes of Kentucky & Indiana
|
| chalcedony |
- cryptocrystalline quartz and much chert
- material agate is made of
- has a great array of colours: blue, gray, black, off-white, purple (deccomposition
of iron-bearing minerals)
- in Hall’s Gap geodes
- sulfide mineralization occurs in chalcedony geode
|
| chert |
- a cryptocrystalline variety of quartz
- occurs as nodules, lenses or layers in limestone and shales
|
| concretion |
- a nodular or irregular concentration of certain authigenic
constituents of sedimentary rocks and tuffs
- developed by the localized deposition of material from solution,
generally about a central nucleus
- are solid, grow from the center outward & are generally
noncrstalline though some crystals have been observed
- concretions are formed by the deposition of distinct minerals,
different from the surrounding rock, very firmly cemented around a
nucleus
- most common cementing materials are calcite, siderite & silica
- parts of plants, animals & well-preserved fossils may be found
at the nucleus
- harder than enclosing rock
|
| enhydro |
- an agate with a visible bubble moving around when tilted
|
| floaters |
- fully formed or doubly-terminated crystals that are unattached to
other crystals
- found in geode cavities & also in vugs such as "Herkimer
Diamonds"
|
| geode |
- hollow, globular bodies
- subspherical shape
- clay film between geode wall & the enclosing limestone matrix
which is like a skin or crust
- an outer chalcedony layer
- an interior drusy lining of inward projecting crystals
- evidence of expansion or growth
- slow deposition of groundwater causes crystals to grow
- estimated it takes 240 million years to form
- geodes form either in sedimentary rock or in basalts/andesites
- hollows appeared in sedimentary rocks either because of animal
burrows, tree roots or mud balls
- word "geode" derived from Latin meaning
"earthlike", rounded shape
- found in USA: Montana, Indiana, Iowa, Illinois, Pennsylvania
(Lancaster Co. goethite geodes), Tennessee (Loretto), Oklahoma
(Comanche County etc.), Michigan (Houghton), Dugway geodes, Colorado
(Chaffee County ,Marshall Pass), Hauser Geode Beds, California
- amethyst geodes from Artigas, Uruguay or near Lajeado, Rio Grande Do
Sul, Brazil
-
|
| industrial geodes |
- a.k.a. garden geodes - found in the garden
- solid all the way through, milky quartz
|
| ironstones |
- large concretions of siderite & calcite found in shales
assoicated with coal beds in Kentucky
- name not much in use now
- collected just south of Holland, Michigan
|
| Keokuk Geode |
- a specific type of geode which can be found in a 45 mile radius of
Keokuk, Iowa in the Tri-State area - Iowa, Illinois, Missouri
- geode is Iowa’s official state rock
- most geodes come from strata of the lower Warsaw Formation, a rock
unit of Mississippian age
- formed in sedimentary rock- shale, shaley dolomite & limestone
- original concretions thought to made of limestone or anhydrite which
then dissolved and was relaced by chalcedony for the skin and
interiors filled in over long periods of time
- source of mineralizing water is speculative as quartz is only weakly
soluble
- predominately quartz crystals
- silicates: crystaline quartz, kaolinite
- carbonates: calcite, aragonite, ferroan dolomite, malachite,
smithsonite, stilpnosiderite
- zinc sulfide
- sulphides: iron pyrite, sphalerite, chalcopyrite, millerite
- sulfates: barite, selenite, jarosite
- oxides: goethite, hematite, pyrolusite
|
| lithophysae |
- hollow, bubblelike structures composed of concentric shells of
finely crystalline alkali feldspar, quartz and other minerals
- found in certain silicic volcanic rocks such as rhyolite and
obsidian
- a.k.a. thunder egg in the USA
- in France, come from the Esterel massif in the south-eastern part of
France
- found in Ploand
|
| nodule |
- small more or less rounded body generally somewhat harder than the
enclosing sediment or rock matrix
- is solid
- agate, jasper & chalcedony form in nodules
- other common minerals that occur in nodules are siderite, gypsum,
calcite, quartz and barite/celestite
|
| rhyolite |
- the aphanitic equivalent of a granite
- thunder eggs form in rhyolitic rocks - Oregon & southeastern
California
|
| septarian nodule |
- septarian nodules that have a hollow cavity are sometimes known as
"septarian nodule geodes" or "geodes"
- Utah septarian nodules formed in ancient sea floor during the
Cretaceous period, 50 - 70 million years ago - filled with calcite -
yellow centres (calcite), brown lines (argonite), grey rock
(limestone), white or clear (barite)
|
| septarium |
- a roughly spheroidal concretion, generally of limestone or
clay-ironstone
- cut into polyhedral blocks by radiating and intersecting cracks
which have been filled (and the blocks cemented together) by veins of
some material, generally calcite
- some contain fossils
- a.k.a. Septarian Nodule, Septarian Boulder, Turtle Stone
- presence of an "Avenue of Entry"
- in southern Illinois, they contain fluorite, sphalerite, witherite
& sometimes galena
- in Utah, the type has a different shape & may be filled with
agate
- thunder egg of Oregon is a septaria - cousin
- most geodes coming from France are in fact septaria (Drome, Ardeche,
Hautes-Alpes, Remuzat)
|
| spherulite |
- a small radiating & usually concentrically arranged aggregation
of one or more minerals generally of spherical or spheroidal shape
- formed by the radial growth of acicular crystals in a rigid glass
about a common centre or inclusion
|
| thunder egg |
- geodelike body commonly containing opal, agate or chalcedony
weathered out of welded tuff or lava
- form only in rhyolitic rocks, almost always in association with
perlite
- exterior surface is "warty" & it has a rind
- interior cross-section commonly exhibits a star-shaped outline
- interiors are filled with chalcedony, agate or opal
- when hollow, thunder eggs are sometimes referred to as geodes
- no thunder eggs exist in the USA east of Colorado except for a
single locality near Grand Marais on Minnesota’s North Shore
- is the common name for lithophysae
|
| turtle stone |
- a septarium with distinctive external markings on the outside
resembling the shell of a turtle
|
| vesicle |
- a small circular enclosed space
- a small cavity in an aphanitic or glassy igneous rock, formed by the
expansion of a bubble of gas or stem during the solidification of the
rock
|
| vug |
- a cavity, often with a mineral lining of different composition from
that of the surrounding rock
- can form spherically, in veins, irregular openings
- vugs do not generally weather out of the host rock
|
Geodes: Some Locations & Types
USA
- Indiana, Washington Co. near Salem Indiana
- clear calcite crystals on pink-orange dolomite
- thin walled
- clear calcite crystals on white quartz
- Iowa, Keokuk – Warsaw Strata
- Keokuk geodes are from a 45 mile radius
- borders Iowa, Illinois & Missouri
- botryoidal chalcedony with calcite cubes
- blue & green chalcedony
- pink calcite crystals
- sphalerite
- quartz iron – cheery orange, quartz oxide – cherry red
- dew drop diamond
- pink dog-tooth
- iridescent brown calcite
- industrial (garden bed) geodes – usually solid all the way through
- interior filled with milky coloured quartz
- found in people’s garden’s
- banded agate lining with crystals
- world-famous locality noted for its millerite in small geodes
- light pink amethyst
- Kentucky, South Fork Creek
- bumpy, brown round geodes in river bed
- stepped calcite, spaghetti, smoky quartz, botryoidal chalcedony
- orange-yellow solids, clear, white
- Sheffler Geode Mine – Keokuk Geodes
- gray, smoth skinned round
- clear quartz crystals
Brazil Rio Grande do Sul
- amethyst
- most fantastic geode known – 33 ft. in length, 16 ˝ ft. wide, 10 ft.
high
- estimated weight of seventy thousand pounds
- lined with purple amethyst
- a piece weighing 400 lbs. is in the Smithsonian Institute
Mexico, Chihuahua
- amethyst quartz, smoky, gray, white, lavender and brown quartz
- along with contrasting crystals of calcite, goethite
Uruguay
- adjoining Brazil, Rio Grande do Sol
Canada
- High Falls, Manitoulin Island, Ontario
|